Disk drives are commonly employed in computer systems to store data. Generally, a disk drive includes several disks that each contain concentric tracks on both of their primary surfaces for storing data, a spin motor that rotates the disks about a central axis at a constant rate, heads that read data from and write data to the disk surfaces (with one head per disk surface), an actuator assembly that radially positions the heads above the desired track, and circuitry such as a preamplifier, read channel, and controller that transfers the data between the heads and a host computer external to the disk drive.
The concentric tracks on separate disk surfaces are typically aligned with each other and form cylinders. That is, each cylinder includes a set of tracks, one track per disk surface, which have the same radius and are aligned with one another. When the actuator assembly radially positions the heads, each of the heads is radially aligned with a track on the associated disk surface within the cylinder.
The host computer delivers access requests to the disk drive whenever the host computer desires to store or retrieve data. To perform an access request, the disk drive first positions the head above the track of the rotating disk specified by the access request. Once the head is properly positioned, the requested data transfer takes place. Usually the head is part of an air-bearing slider that is aerodynamically designed to ride above the disk surface on a thin cushion of air created by the rotation of the disk, although contact and pseudo-contact sliders are also known in the art.
Writing is performed by delivering a write signal with polarity-switching write current to the head while the head is positioned above the desired track. The write signal creates a variable magnetic field at a write gap of the head that induces magnetically polarized transitions into the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by the head sensing the magnetically polarized transitions on a track. As the disk spins below the head, the magnetically polarized transitions on the track induce a varying magnetic field in the head. The head converts the varying magnetic signal into an analog read signal that is amplified by the preamplifier and then delivered to the read channel. The read channel converts the analog read signal into a digital signal that is processed and provided to the host computer.
Disk drives typically include servo areas and user data areas. The servo areas facilitate positioning the head over the desired location, and the user data areas store data provided by the host computer that can be subsequently accessed by the host computer. In an embedded servo format, each of the tracks includes numerous sectors, and each of the sectors includes a servo area and a user data area. The servo area typically includes separate fields for automatic gain control, synchronization, address information, and servo bursts. The servo bursts are read by the associated head and delivered to a servo control unit which sends position error signals to a voice coil motor of the actuator assembly for positioning the associated head with respect to the track. In a dedicated servo format, one disk surface contains the servo areas for all the other disk surfaces.
The manufacture of a disk drive, regardless of whether an embedded or dedicated servo format is employed, typically includes writing the servo areas on a disk (referred to as servo track writing) and then testing the disks to identify defective sectors where user data cannot reliably be stored. Typically, less than a threshold number of defective sectors are tolerated and accommodated by employing spare sectors in place of the defective sectors. The defective sectors are identified in a bad sector bit map and are no longer utilized. However, if the number of defective sectors exceeds the threshold number then the disk drive is rejected and removed from the assembly line.
The conventional approach for identifying defective sectors involves writing a test pattern to all the user data areas and reading all the user data areas to determine whether the read signals contain errors. The reading typically includes several read operations to improve reliability. If the read signal for a given sector contains more than a threshold number of errors, the sector is flagged as defective.
With conventional flaw detection procedures, the test patterns are written and subsequently read one head at a time. For example, in a disk drive with first and second heads associated with top and bottom surfaces of a disk, the heads are positioned about a given cylinder, the first head writes the test pattern to each sector on a track on the top surface of the disk, then the second head writes the test pattern to each sector a track on the bottom surface of the disk, then the first head reads the test pattern from each sector on the track on the top surface of the disk, and then the second head reads the test pattern from each sector on the track on the bottom surface of the disk. Alternatively, the first head writes the test pattern to each sector on a track on the top surface of the disk, then the first head reads the test pattern from each sector on the track on the top surface of the disk, then the second head writes the test pattern to each sector on a track on the bottom surface of the disk, then the second head reads the test pattern from each sector on the track on the bottom surface of the disk. In either case, the process is repeated cylinder by cylinder until all sectors on the top and bottom disk surfaces have been tested. Unfortunately, these techniques are very time consuming. Moreover, disk drives that employ more disks require correspondingly greater test time. For instance, a disk drive with three disks and six heads requires three times as much test time as a disk drive with one disk and two heads.
Flaw detection is a limiting factor in the throughput of disk drives that a disk drive manufacturing facility can produce in a given amount of time. This limitation can be addressed by adding more flaw detection stations to the assembly line, however this approach is inefficient and expensive. Accordingly, a need exists for an improved technique for providing rapid flaw scans in disk drives.